The Cornish bit is a useful lathe drill, apparently derived from the boring bar, fig. 514, page 569, and like it, is adapted to holes of certain fixed diameters. As seen in fig. 1018, the stock which is cylindrical throughout, (except where it is square for the hook wrench,) is enlarged at the one end, and has a diametrical mortise, fitted with a cutter c, notched out at the one end to embrace the flattened sides of the bar, and secured with the wedge w, as in fig. 514. But as the Cornish bit is used in the lathe, and is therefore only supported by the work at the one end, and the popit-hesd at the other, a bearing piece b is fitted in a longitudinal chamfer groove on the under side of the stock, as seen in the end view, in order to keep it central The three edges, of the cutter and bearing piece respectively, are all turned in their places. The cutter is bevelled and rounded so as to cut at the front only, after which the parts are hardened;

Fig. 1018.

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The Cornish bits are not made for holes smaller than about 1 1/2 inch in diameter, and by means of additional cutters and bearing pieces, every stock may admit of an increase of size of fully one-half its minimum diameter, or say from about l 1/2 inches, and larger sizes in proportion. This is a very effective tool, and is in general use amongst engineers.

Note BG. - To follow the text, pages 564 to 567. (Messrs. Maudslays' boring bits, with loose cutters for boring the bosses of wheels, pumps, and small steam cylinders in the lathe.) Boring bits of the kind represented in fig. 1019, ranging from about 3 to 12 inches in diameter, and with a power of variation in size, were many years back introduced by Messrs. Maudslays, and employed for boring the bosses of wheels, small cylinders, pumps, etc. The stocks of the smaller sizes of these tools are made in wrought-iron, those of the larger in cast-iron, the cutters rest in contact with a fillet made on the stock exactly at right angles to the axis, and are held down by screws which pass through mortises in the cutters, to enable these to be set out to various diameters. The bearing pieces beneath, although generally fitted in a chamfer groove, are also made to admit of packing pieces by which they may be set out. to make the three points of bearing to fall in a circle of the exact diameter of that to be bored. The larger of these tools, the cutters of which are 3/4 inch thick, are now very much less used, since the boring bars with sliding heads or blocks, referred to in pages 569 to 572 of the text, have obtained such general employment for boring cylinders and pumps.

Fig. 1019.

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Note BH. - To follow the text, pages 564 to 567. (Mr. Stivens' Registered Lathe Drill.)

This instrument is represented in perspective in fig. 1020, and in plan with the top plate removed in fig. 1021, it has two cutters which are adjustable for various diameters; the tool is intended to be used after the manner of figs. 509, 510, and 511, page 565, that is with the loop fig. 511. The two cutters c c, lie in oblique grooves, the ends of which are at an angle of 45 degrees, and between the cutters u placed a wedge w w, the long shaft whereof extends through the entire length of the drill shaft, and has the hollow center for receiving the pressure of the popit-head. When this long wedge is set forward, by a tail screw and nut, (not represented,) it throws out the two cutters in any required degree, so that the bit for holes of one inch in diameter, may be thus enlarged for any size not exceeding about 1 1/4 inch, and so with the larger tools.

The cap piece, or plate p p, which is represented removed, and is attached by three scrcws, has a shallow circular recess within which the two pins fixed to the cutters are loosely contained, to prevent them from being accidentally lost. The cutters are rounded at the ends, and sharpened precisely like the bit fig. 509, and the two edges of the shaft from a to b, are made symmetrical and with rectangular edges, in order to stick into the aides of the loop inch as fig. 511, from which this drill receives its axil guidance, in the manner already explained on page 566; but it appears objectionable that the guiding loop should from necessity bo so far removed from the cutting edges of this expanding drill, which is proposed to be mode as large as eight inches in diameter.

Figs. 1020.

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Note BI. - To follow the pages 564 to 567. (Mr. Kittoe's Expanding half-round Bit.)

In this instrument, three parts instead of two only arc made to adjust radially and equally; from the one point only being sharpened, and from there being a bottom bearing upon the surface of the hole that is bored, the instrument is used in all respects as the common half-round bit, and without the necessity of the loop or guide, (fig. 511, page 565), required with Mr. Stirens' expanding drill, and of which latter Mr. Kittoe bad not the least knowledge when be constructed the present tool.

Fig. 1022 is the perspective view of the boring bit when in condition for work, fig. 1023 is a section of the same through a horizontal plane; and a to k; fig. 1024, are the parts shown separately, the same letters being attached to the some parts throughout The part from c to g is of brass, and contains all the mechanism, g to h is an iron rod screwed into the brass to serve as the shaft of the tool, and which is made of any required length. The portion from c to e is constructed in two pieces, which separate nearly on their diameter, and are united jointly by steady pins and the screwed nut d e, the division being made for the purpose of introducing the two bits a a, one of which only is made to cut; b the bottom bit, is inserted in a similar but vertical cleft.

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